Various types of digital exposure apparatus have been developed, one type being an exposure apparatus utilizing a fluorescent head. This fluorescent head has a cathode electrode and an anode electrode both sealed within an evacuated case, and dots formed by phosphors are arranged in a line on the anode electrode. When a voltage is applied to the cathode electrode, electrons are emitted from the cathode electrode and, when the emitted electrons impinge on the anode electrode, the phosphors are excited to produce light. The exposure apparatus utilizing such a fluorescent head outputs the thus produced light to expose a photosensitive material to print an image thereon. In the exposure apparatus utilizing such a fluorescent head, when many dots are illuminated at a time, more electrons are emitted from the end portions of the cathode electrode than from the center portion thereof; as a result, of the dots arrayed in a line on the anode electrode, the dots at the end portions tend to become brighter than the dots in the center portion.
However, when a large current flows to the end portions of the cathode electrode, the temperature at the end portions of the cathode electrode increases. When the amount of oxides scattering from the end portions increases, the scattering oxides adhere to the phosphors, and the efficiency of light emission drops. As a result, there arises the problem that, as the accumulated time of light emission increases, the dots in the end portions of the dot array become darker. To solve this problem, there has been proposed a method that uses a storage means for storing a plurality of data tables for controlling the light emission of each individual dot and a switching means for switching between the plurality of data tables, and that corrects for the variations in the amount of light between the end and center portions of the dot array on a dot-by-dot basis by switching between the data tables (for example, patent document 1: Japanese Unexamined Patent Publication No. H07-256921).
According to the fluorescent head disclosed in the patent document 1, since the variations in the amount of light, occurring between the end and center portions of the dot array as the accumulated time of light emission increases, can be corrected for on a dot-by-dot basis, uniform exposure can be achieved even when the accumulated time of light emission increases. Furthermore, as the adjustment of print density can be accomplished by switching between the data tables, there is no need to provide a drive voltage adjusting circuit, etc. for adjusting the print density.
Other types of digital exposure apparatus include, for example, an LED exposure apparatus which comprises light-emitting diodes (hereinafter abbreviated LEDs) substantially arranged in a line, and which outputs a photographic image by controlling the amount of light emission of each LED in accordance with image data. The LEDs used as light sources in this type of exposure apparatus have the problem that the amount of light emission and their spectral characteristics vary depending on the ambient temperature. There is the further problem that the spectral sensitivity characteristics of the photosensitive material to be exposed also vary depending on the ambient temperature. To solve these problems, there has been proposed an exposure correction method that controls the LED drive current by taking into account the amount of light emission and the spectral characteristics of the LEDs in relation to the temperature and the spectral sensitivity characteristics of the photosensitive material, thereby maintaining exposure conditions constant irrespective of variations in temperature (for example, patent document 2: Japanese Examined Patent Publication No. H04-046472).
In the exposure correction method disclosed in the patent document 2, correction coefficient tables are constructed by taking into account the amount of light emission and the spectral characteristics of the LEDs in relation to the temperature and the spectral sensitivity characteristics of the photosensitive material; then, an appropriate one of the correction coefficient tables is selected in accordance with the temperature, and the input image data is multiplied in a multiplier by a correction coefficient to correct the image data, the method thus aiming to stabilize the exposure conditions.
As another type of exposure apparatus, an exposure apparatus has been developed that outputs a photographic image by controlling the exposure time in accordance with grayscale data by means of an optical shutter whose elements are arranged in a line. Generally, a PLZT device or a liquid crystal shutter or the like is used as the optical shutter in this type of exposure apparatus; however, because of the presence of a rise time, i.e., the time taken to achieve a transmissive state after applying a voltage for turning on the optical shutter, and the presence of a fall time, i.e., the time taken to achieve a non-transmissive state after applying a voltage for turning it off, the relationship between the ON voltage application time and the amount of exposure light is nonlinear. Furthermore, the relationship between the amount of exposure light and the exposure density (i.e., the print density) on the photosensitive material is also nonlinear. There has therefore been the problem that correct grayscale cannot be reproduced even if the photosensitive material is exposed to light from a photographic image or the like by controlling the optical shutter proportionally to the grayscale data.
To solve this problem, an exposure apparatus has been proposed that comprises a converting means for correcting the exposure density having a nonlinear relationship to the grayscale data, and an exposure control means for controlling the exposure time based on the corrected exposure time data output from the converting means (for example, patent document 3: Japanese Patent No. 2956556). The exposure apparatus disclosed in the patent document 3 aims to achieve correct grayscale reproduction by using a conversion table, which defines a correspondence between the grayscale data and the corrected exposure time data for obtaining the exposure density (or the amount of exposure light) proportional to the grayscale data, and a referencing means for referencing the conversion table and outputting the corrected exposure time data that matches the input grayscale data.
There has also been developed an exposure apparatus of the type that forms an image by an electrophotographic method by illuminating a photoconductor with laser light. This laser exposure apparatus has had the problem that the image density changes with temperature and humidity because the photoconductor has a temperature and humidity dependence.
To solve this problem, it has been proposed to store as many look-up tables as there are temperature/humidity conditions, and to compensate for changes in temperature and humidity by switching between the look-up tables as the temperature and humidity change (for example, patent document 4: Japanese Unexamined Patent Publication No. H05-197262).